CHAPTER X.

CONDENSATION OF ELECTRIFICATION.

=EXPERIMENT 82. To find whether a large surface will hold more electrification than a small one.=

_Apparatus._ The insulating table (for details, see Exp. 64); a large tin basin or pan (not furnished); the electrophorus (Exp. 68).

=175. Directions.= (A) Test the electrophorus and be sure that it is working properly.

(B) As in Exp. 70, see how many good sparks I T will take from E C (which should be recharged at each trial) before the potential of I T is raised so that it equals the potential of E C.

(C) Carefully set the basin or pan upon I T, then count the number of good sparks you can pass to it from E C (recharged at each trial). Compare the number of sparks necessary to raise the potential of the large surface until it equals that of E C, with the number found in part (B).

_=176. Electrical Capacity.=_ It takes more heat to raise the temperature of a gallon of ice-water to the boiling point, than it takes for a quart of ice-water. You have just seen that a large insulated surface will take more sparks from a charged body than a small one, before its potential is raised to that of the small one, and to that of the charging body. We say that a large surface has a greater _capacity_ than a small one, the shape and other conditions being the same.

=EXPERIMENT 83. To find whether the capacity of a given conductor can be increased without increasing its size.=

_Apparatus._ Fig. 52. Insulating table. I T (Exp. 64); the extra ebonite sheet, E S (No. 27); the complete flat box, F B (No. 40, 41); the charged electrophorus cover, E C (Exp. 68). Arrange, as shown, I T being insulated from the earth by E S. F B should rest upon a wooden table or other large conductor.

=177. Directions.= (A) See how many good sparks I T will take from E C. Re-charge E C at each count, and note the relative sizes of the sparks.

(B) Discharge I T by touching it with your knuckle.

_=178. Condensation; Condensers.=_ As I T easily held more sparks than it would take before (Exp. 70), we say that its _capacity_ has been increased. Its potential didn't increase, because that could not get greater than the potential of E C, the charging body. To describe this state of affairs, we say that the electrification was denser than before, and that it was _condensed_. The _capacity of I T was greatly increased by the presence of another conductor, F B, insulated from I T, but "grounded_." Such a combination, 2 conductors, with a dielectric between them, is called a condenser.

A condenser can hold much more electrification at a certain potential than an equal amount of surface can hold when not properly arranged. We might call a condenser a storage battery for static electricity. The capacity of a condenser depends, among other things, upon the area of the conducting surfaces, and upon the thickness and nature of the dielectric. Among the various forms of condensers may be mentioned the Leyden jar, and the fulminating pane.

=179. The Leyden Jar= consists of a wide-mouthed glass jar, with tin-foil pasted upon the inside and outside to within 2 or 3 inches of the top. The inner coat or conductor is connected to a knob or ball at the top by means of a chain. To charge the jar, the outer coat is connected with the earth by holding it in the hand, or by resting it upon a table while the electrification is passed to the knob. A _Leyden Battery_ consists of 2 or more connected jars, the object being to increase the area of the surface. The jar is discharged by touching one end of a _discharger_ (§ 188) to the outer coat, and swinging its other end over to the knob, when a bright spark will pass between the knob and discharger. (See Exp. 86.)

=180. Fulminating Panes=, or Franklin's Plates, are practically the same as a Leyden Jar. The tin-foil, however, is pasted upon the opposite sides of a pane of glass, a margin of about an inch being left all around. One side of the pane is charged, and takes the place of the inside coat of the jar. The other side is grounded. The pane is discharged by connecting the two sheets of foil.

=181. Induction Coil Condensers= consist of sheets of tin-foil separated by sheets of paraffined paper, which act as the dielectric. (See Induction Coils.)

=182. Submarine Cables=, with the surrounding water, act like condensers, the result being that the condensing effect slows up the electric current and retards the signals. These make a condenser of enormous capacity. The wires inside form one conductor, and the water the other, while the insulation around the wires forms the dielectric.

=EXPERIMENT 84. To study the condensation of electrification.=

_Apparatus._ Same as in last experiment, but arrange so that F B and I T shall be near each other at one side; that is, so that the edge of E S shall be even with the edges of the two tins.

=183. Directions.= (A) Pass good sparks to I T from the charged E C until something happens. Watch the side where I T and F B are near each other.

_=184. Discussion.=_ We may say that the electrification was condensed, in this experiment, until the charge became so great that the _condenser_ suddenly discharged itself. Condensers may be made in many ways, but they all consist of 2 conductors, with a dielectric between them. One conductor is insulated, and receives the charge; the other conductor is grounded.

=EXPERIMENT 85. To study the action of the condenser.=

_Apparatus._ Fig. 53. The insulating table, I T; ebonite sheet, E S (No. 27); flat box, F B, complete (Nos. 40, 41); the electrophorus (Exp. 68). Note that this is really the same apparatus as that just used; both conductors of this condenser, however, are insulated and reversed in position.

=185. Directions.= (A) See that your electrophorus works properly, then find out how many good sparks you can pass from E C to F B, recharging E C each time. Note the relative sizes of the sparks, and compare the result with the number taken by the condenser in the last experiment.

(B) When F B seems to be fully charged, touch I T with your knuckle. (From your study of induction what should be the result?)

(C) Now see if F B will again take good sparks from the charged E C. Pass sparks to F B until it seems fully charged.

(D) Again touch I T, then repeat (A) and (B) several times, until a bright spark passes from F B over the edge of E S to I T.

_=186. Discussion.=_ The action of the condenser, as clearly shown, depends upon induction. You should now be able to explain and show by diagram the different steps.

E C was positively charged (Exp. 80). This also charged F B positively by contact. F B acted inductively through the dielectric, E S, drawing up _some_ of the - in I T, and repelling _some_ of the +. As I T was insulated, this free + electrification could not escape. Before we touched I T, its + and - electrifications, although partially separated, were struggling against this inductive action; and, on account of their strong attraction for each other, our efforts to charge the condenser were retarded. Upon touching I T the free + escaped to the earth. (This was the cause of the spark.) This left _some_ - electrification bound on the underside of E S, and some + bound on the upperside of E S. The capacity of F B was increased by this process, as the + already put into it was very much occupied by the attractions of the induced - just under E S. As more + was given to F B, more - was drawn up under E S and more + was pushed out of I T. This action went on until the two conductors were strongly and oppositely charged. This action goes on continuously when the lower conductor is grounded. The spark between the tins was due to the rushing together of the + and - electrifications; it showed that there was a _momentary current of electricity_.

=EXPERIMENT 86. To study the effect of electrical discharges upon the human body.=

_Apparatus._ The condenser (Fig. 52), with E S centrally placed so that the apparatus cannot discharge itself; the hairpin discharger, H P D (No. 48); the electrophorus.

=187. Directions.= (A) Charge the condenser (Exp. 83) with 10 good sparks from E C, then touch I T (Fig. 52).

(B) Recharge the condenser with 10 sparks, then touch F B. Discharge it by again touching I T as in (A).

(C) Recharge with 10 sparks; then place your thumb against F B, and quickly swing the first finger of the same hand over to I T, and get a slight shock.

(D) Recharge with as many sparks as you think you can stand.

(E) Instead of using your hand to discharge the condenser, try the bent hairpin. Keeping one end against F B, swing the other end over near I T.

_=188. Shocks; Dischargers.=_ The two conductors being oppositely charged in the condenser (Exp. 85), it is only necessary to place some conductor between them to allow the charges to rush together. Any conductor so used is called a _discharger_. The hand carried the whole current which caused the _shock_. When I T was touched first, the current was obliged to pass through your body, through the floor, and up the table-legs into F B. Always touch the "grounded" conductor first with the discharger, so that you will get a good spark and _not_ a shock.

=EXPERIMENTS 87-88. To show the strong attraction between the opposite electrifications in the condenser.=

_Apparatus._ Flat box, F B (Nos. 40, 41); sheet of glass, G (No. 38); electrophorus (Exp. 68). The two parts of F B are used for the conductors of the condenser (Fig. 54) for the sake of lightness. The bottoms should be next to the glass, which is used for the dielectric on account of its stiffness. The lower tin should rest upon the table. The glass should be perfectly clean and dry (hot).

=189. Directions.= (A) Charge the condenser with 15 or 20 good sparks from E C.

(B) Lift the condenser by one corner of G (Fig. 54), being careful not to discharge it. Explain why the lower conductor follows the glass.

=EXPERIMENT 88.=

=190. Directions.= (A) Charge and lift the condenser as just explained (Exp. 87). Fig. 54.

(B) With your right hand touch the upper tin alone, then the lower tin alone.

(C) Touch both tins at the same time, and note the action of the lower one.

_=191. Discussion.=_ This clearly shows how strongly the two electrifications are _bound_ in the condenser. Each refuses to escape to the earth, but they instantly rush together at the first opportunity. The dielectric may be shattered in a very heavily-charged condenser by this strong attraction.

=EXPERIMENT 89. To show how the condenser maybe slowly discharged.=

_Apparatus._ Fig. 55. The condenser (Exp. 83); the carbon electroscope with support (Exp. 58); the electrophorus (Exp. 68).

=192. Directions.= (A) Charge the condenser by means of the electrophorus; then hang the carbon so that it can swing between the upper conductor and E C placed as shown.

_=193. The Electric Chime.=_ The charging and discharging of the carbon being rapid, it acts like a _chime_ as it taps against the tins.

=EXPERIMENT 90. To ascertain the location of the charge in the condenser.=

_Apparatus._ The condenser, consisting of flat box, F B (Nos. 40, 41); ebonite sheet, E S (No. 27); insulating table, I T (Exp. 64); (when charging, arrange as in Fig. 52.); the electrophorus; hairpin discharger, H P D (No. 48).

=194. Directions.= (A) Charge the condenser with 15 or 20 good sparks from E C.

(B) Lift I T away from E S by its insulating handle, and set it upon the table. (It may be necessary to hold E S down.)

(C) Lift E S directly up and away from F B. (Lift by 2 corners; do not scrape E S along on F B; do not allow E S to touch your clothing.)

(D) Replace E S and then I T by its handle quickly, making the condenser complete again.

(E) With H P D see if the condenser still holds a charge. Touch F B first (Exp. 86).

_=195. Discussion.=_ As the _conductors_ were completely discharged, being left for a few moments upon the table, it is evident that the opposite electrifications must reside in and upon the _dielectric_. The conductors allow an even and _rapid_ discharge from all parts of the dielectric at the same time. The dielectric is considerably strained when a condenser is heavily charged. This strain, caused by the attraction of the opposite electrifications, may be great enough to break or puncture the dielectric.

=EXPERIMENT 91. To find whether any electrification remains in the condenser after it has once been discharged.=

_Apparatus._ The condenser (Fig. 52); the electrophorus (Exp. 68); hairpin discharger, H P D.

=196. Directions.= (A) Thoroughly charge the condenser.

(B) Discharge it with H P D, being sure to touch F B first, and to touch I T for an instant while H P D is against F B.

(C) After a few moments use H P D again, and see if you get a slight spark.

_=197. Residual Charge.=_ The two electrifications on the opposite sides of the dielectric have such an attraction for each other, when the condenser is charged, that they seem to penetrate, or soak into, the dielectric. These do not completely soak out again at the discharge. The small amount left is called a _residual charge_.

=EXPERIMENT 92. To study successive condensation; the chime cascade.=

_Apparatus._ Fig. 56. This really consists of two condensers, joined by a wire. The upper condenser consists of T F B (No. 41), E S (No. 27), and the insulating table, I T. (See Exp. 64.) The lower condenser consists of the cover of the tin box, C T B (No. 47), the sheet of glass G (No. 38), and B F B (No. 40). The tin box, T B (No. 47), is placed under this to raise it, simply. A wire or hairpin, H P, is hung upon the edge of T F B, its lower end being inside of C T B and not quite touching it. This acts like a pendulum, which is to swing to C T B at the proper time. The source of electrification is E C.

=Note.= You have learned that in charging the condenser with the positively charged E C, + electrification is driven from F B into the earth. Can we use this to charge a second condenser?

=198. Directions.= (A) Pass 15 or 20 good sparks from E C to the under side of I T (Fig. 56), noting the action of H P.

(B) Hold E C in the hand, and, with its insulating handle, poke H P away from the condensers. Do not discharge them.

(C) With H P D test the lower condenser for a charge, touching T B first.

(D) With H P D touch T F B first (why?), and discharge the upper condenser.

_=199. Discussion.=_ A long row of condensers may be charged in this way. There is no advantage in it, as the electrification is merely divided between them. How can two condensers be joined to get the advantages of a large surface?